Neuroendocrine cells secrete hormones which control endocrine function and neurotransmission. This phylogenetically old system has come under direct scrutiny because of the refinement of procedures which allow detection of minute quantities of releasing hormones and visualization of neurons transporting them. The proposed research will use electron microscopy, immunocytochemistry and radioimmunoassay to identify the complete LH-RH system including cell bodies, processes and sites of termination. Particular emphasis will be placed on cell bodies in the prechiasmatic area which may influence behavioral as well as the release of gonadotropic hormones and on fibers and terminals in the arcuate-median eminence region. (2) How extrahypothalamic systems involved in sensing internal and external environmental conditions influence neurons synthesizing LH-RR will be specified after their morphological interrelationships have been identified. (3) Neuronal cells which sense circulating levels of steroid hormone and mediate their negative and positive feedback actions on the gonadotropic systems are to be studied. (4) A description will be provided of changes in the LH-RH system associated with development, from the prenatal period to senescence, with natural and experimentally produced sexual dimorphisms. (5) Coordinated with this endeavor will be identifying neurons synthesizing and releasing growth hormone-release inhibiting hormone. Thus sexual behavior, physical growth and critical stages in development will be interrelated with two brain neurosecretory systems. Reproductive behavioral capabilities will be correlated with concentrations and locations of LH-RH in the nervous system.